1. Field of the Invention
The object of the invention is to propose a system for accurately locating the position of a vehicle, in particular an aircraft. Air traffic control requires the position of each aircraft to be known. It is known in the art to use radar and transponders on board aircraft, each transponder responding to a radar pulse by transmitting the identity of the aircraft. A first drawback of radar is that not all of the surface of the earth is covered. A second drawback of radar is its relative inaccuracy. With air traffic constantly increasing, it is necessary to improve the accuracy with which the position of aircraft is determined in order to be able to reduce the spacing between aircraft. The maximum accuracy of radar is limited by various phenomena and decreases with distance, which makes it necessary to consider using other systems to determine the position of each aircraft with greater accuracy.
2. Description of the Prior Art
It is known in the art to use a GPS (Global Positioning System) receiver on board aircraft to determine the position of the aircraft with very great accuracy by a method known in the art based on the reception of a plurality of signals transmitted by satellites NV of the Navstar constellation. It is known in the art to transmit the position of the aircraft automatically by radio to a ground station and then to an air traffic control center via a terrestrial transmission network dedicated to this service. However, this system has two drawbacks:
a dedicated network is very costly; and
the accuracy of the position transmitted is degraded by the fact that the transmission time between the aircraft and the control center is not perfectly constant: fluctuations in the transmission time correspond to position differences which are proportional to speed and which are therefore not negligible if an aircraft is travelling at 1000 kph, for example.
The invention provides an onboard system in a vehicle for locating the vehicle accurately, the system including:
means for determining the position of the vehicle from radio signals from a constellation of navigation satellites, and
means for periodically transmitting to a control center the position thus determined, a time label supplied by the constellation of navigation satellites and the identity of the vehicle.
The invention also provides a central system for accurately locating at least one vehicle provided with an onboard system of the above kind, the central system including means for calculating the position at the current time as a function of the position and the speed vector of the vehicle and the time label transmitted by the vehicle.
The above systems determine a position periodically with great accuracy because the time label supplied by the navigation satellite constellation enables fluctuations in the transmission time between the vehicle and the air traffic control center to be compensated when the position is received in the air traffic control center.